In conventional evaporating plants wherein a liquid is concentrated by evaporation in one or more stages or effects, the vapor from the last stage is usually condensed by indirect contact with cooling water in a condenser.
The present invention is directed to a method for concentrating a liquid by evaporation resulting in high thermal efficiency by utilizing the heat energy of the vapor leaving the evaporator instead of removing heat energy from the evaporating system to an external cooling water system.
It is accordingly an object of the present invention to improve the efficiency and to lower the energy consumption of air conditioning systems using absorption technology.
The present invention is particularly useful in the dehumidification of gas such as air by a water-absorbing salt solution which will be diluted by absorbing moisture from the gas. To enable the spent absorption liquid to be reused it is reconcentrated by evaporation.
According to one embodiment of the present invention, air is dehumidified by direct contact with a water-absorbing liquid. As the water absorbing liquid is used an aqueous solution of an easily soluble salt such as potassium acetate, sodium acetate, potassium carbonate, calcium chloride, lithium chloride or lithium bromide or the like or mixtures thereof. These concentrated salt solutions exhibit great affinity to water and consequently, exhibit low water vapor pressure above the solution.
If air is brought into contact with such a solution at a certain temperature and a certain relative humidity the solution will absorb water vapor from the air as long as its concentration results in a lower water vapor pressure than that present during the state of equilibrium.
When air is dehumidified by absorption of water vapor, the absorption liquid will become increasingly diluted by the absorbed water. As the only volatile component of the absorption liquid is water, the solution can be regenerated by evaporation. This is usually carried out by heating absorption liquid to a temperature at which the water vapor pressure thereof exceeds the atmospheric pressure thus causing the water to evaporate. The boiling point elevation of the concentrated saltwater solution suitable for absorption purposes is high. Generally, the dilution of the absorption liquid by absorption of water vapor is relatively small, and consequently evaporation in more than one stage or effect is usually not feasible so that the diluted absorption liquid is usually regenerated by evaporation in a single stage evaporator.
To regenerate the absorption liquid in an evaporator an amount of energy corresponding to the heat of vaporization is required. Additional energy is needed to heat up the liquid to the boiling temperature thereof and to compensate for heat losses and the like.
According to a preferred embodiment of the present invention, the heat energy of the vapor leaving the evaporator is used to evaporate water from the absorption liquid. This is accomplished by bringing the vapor into indirect contact with the absorption liquid before and/or after the absorption liquid is concentrated in the evaporator. The vapor exiting from the evaporator is brought into contact with the one surface of a heat exchange element while the absorption liquid is brought into contact with the other surface of the heat exchange element. The absorption liquid is caused to flow down preferably in form of a uniform thin film over the surface of the heat exchange element. In addition air is caused to flow in contact with the absorption liquid to lower the water vapor pressure above the absorption liquid thus enhancing the evaporation of water from the absorption liquid. The air will be saturated by water vapor and the vaporization heat is removed from the surface of the heat exchange element.
By condensing the vapor from the evaporator by means of the absorption liquid which thus will be concentrated before th evaporator and/or after the evaporator, a higher coefficient of performance as well as significant energy savings are obtained. An additional advantage of the invention is that the need for an external cooling water system is eliminated.
The present invention thus achieves with one evaporator an effect similar to evaporation in two stages at considerably lower specific energy consumption. Although only one evaporator is shown in the accompanying drawing, it will be understood that more than one evaporator can be used. If the number of evaporators used in the practice of this present invention equals n, the process of the invention effectively results in evaporation corresponding to n+1 stages or effects.
For a more complete understanding of the present invention and for further objects and advantages thereof, reference may now have to be taken to the following description in conjunction with the accompanying drawing.